This invention relates to a heater assembly for heating blow molding plastic preforms in preparation for blow molding thereof as containers.
Injection molded preforms in one type of processing are cooled after the injection molding and subsequently heated in preparation for blow molding as containers. Such preforms conventionally have an elongated shape with a closed end and an open end that is injection molded to the dispensing end configuration of the container, normally with a threaded construction for securing a closure cap utilized to seal and selectively allow opening of the container for dispensing of its contents.
Conventional ovens for heating blow molding plastic preforms include a plurality of heater assemblies spaced along a conveyor on which the preforms are conveyed adjacent the heater assemblies, usually with the preforms in an upside down orientation with their open ends positioned downwardly and their closed ends projecting upwardly, and with the preforms being rotated about associated horizontal axes so as to provide uniformity of the heating provided by the heater assemblies. Each heater assembly conventionally includes a plurality of elongated heaters that each have an elongated bulb for heating the conveyed preforms. Usually such bulbs have a white oxide coating on the side thereof that faces away from the preform so that radiation is reflected from that coating back toward the preform in an attempt to provide efficiency in the heating. However, such coatings cause substantial heating of the bulbs and are not efficient in reflecting infrared light which is more effective in uniformly heating the preforms than longer wave light. More specifically, the infrared light penetrates the outer surface of the preforms to provide more uniform heating of the interior so that heating can be accomplished without overheating the outer surface.
Prior art patents that involve heating of blow molding plastic preforms include U.S. Pat. No. 5,256,341 Denis et al. and U.S. Pat. No. 5,681,521 Emmer et al.
An object of the present invention is to provide an improved heater assembly for heating blow molding plastic preforms in preparation for blow molding of the preforms.
In carrying out the above object, the heater assembly of the invention includes a housing for mounting adjacent a conveyor by which the preforms are conveyed along a path of conveyor from a supply of the preforms to a blow molding machine where the heated preforms are blow molded. A plurality of elongated heaters are mounted by the housing extending along the-path of conveyance in a parallel extending relationship to each other. Each heater includes a pair of mounts for mounting an associated elongated bulb that extends along the length of the heater and has an element that is energized to irradiate the conveyed preforms. Each elongated heater also includes an elongated parabolic reflector that is closely spaced from the associated elongated bulb to reflect radiation thereof to irradiate the conveyed preforms. The element of the associated bulb of each heater is located at the focus of the parabolic reflector such that the parabolic reflector reflects radiation rays in a generally parallel relationship to each other in order to provide controlled heating of the conveyed preforms.
A blow molding plastic preform also includes a window that is located between the heaters and the conveyed preforms. This window is preferably made of quartz and transmits infrared radiation but blocks longer wave radiation so as to provide more uniform heating of the conveyed preforms.
The elongated heaters of the heater assembly are mounted by the housing in a vertically stacked relationship with the parabolic reflectors of the heaters reflecting radiation rays in a horizontal orientation to provide the heating of the conveyed preforms. The heater assembly also includes a lower radiation shield that shields the lower extremity of each conveyed preform from irradiation.
It is also preferably for the heater assembly to include a lowermost heater whose bulb element is spaced closer to the bulb element of the next higher heater than the spacing between the bulb element of the uppermost heater and the bulb element of the next lower heater.
The objects, features and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.